I found out at 15 weeks a girl. You can tell it's just not always accurate. Just ask to make sure further along. That's why they usually wait till 20 weeks to make sure.
They all look like boys at 13w. I'm surprised they even mentioned gender tbh. I definitetly wouldn't start buying anything gender specific until you've had a scan later than 16w.
I wouldn't buy anything just yet. Genitals are still developing at that point. I would wait til the anatomy scan before buying anything.
I found out at 14 wks with my boys. Later with my girls. But my boys are on the larger side, there was no mistaken